Environmental concerns and the search for savings in fossil energy resources have led to the development of vehicles with electric motors. However, the latter are limited in terms of power and range, and require a very long battery recharging time.
Hybrid engine systems remedy these drawbacks by utilizing an electric motor and a standard thermal internal combustion engine in series, in parallel or in combination. In a hybrid vehicle, starting is ensured by the electric motor. Up to a speed of the order of 50 km/h, it is the electric motor which provides the driving power of the vehicle. From the moment that a higher speed is reached or a high acceleration is required, the thermal internal combustion engine takes over. When the speed reduces or during vehicle stops, the thermal internal combustion engine stops and the electric motor takes over. Thus, the thermal internal combustion engines of hybrid vehicles are subjected to a significant number of stops and restarts compared to a thermal internal combustion engine of conventional vehicles.
Moreover, certain vehicles are equipped with the “Stop-and-Start” system also called automatic stop and restart device. These vehicles are generally considered as “micro-hybrid” vehicles. In fact, these vehicles are equipped with a thermal internal combustion engine and a starter-alternator or a heavy-duty starter which ensures the stopping and restarting of the thermal internal combustion engine when the vehicle is at a standstill. The thermal internal combustion engines of micro-hybrid vehicles equipped with the “stop-and-start” system, like the thermal internal combustion engines of hybrid vehicles, are therefore subjected to a significant number of stops and restarts compared to thermal internal combustion engines of conventional vehicles.
Thus, over its lifetime, the thermal internal combustion engine of a hybrid vehicle or micro-hybrid vehicle is subjected to a much larger number of stops and starts than that of a standard vehicle. This potentially produces specific wear problems for the thermal internal combustion engines of hybrid and micro-hybrid vehicles, in particular over the long term. These specific wear problems are in particular visible on the connecting rod bearings. There is therefore a need to develop novel lubricant compositions allowing the reliable operation of the internal combustion engines of hybrid and micro-hybrid vehicles equipped with the Stop-and-Start system, and in particular capable of reducing wear, in particular wear on the bearings, in particular wear on the connecting rod bearings in the thermal internal combustion engines of said vehicles.
Surprisingly, the Applicant has found that the use of certain polyalkylene glycols in the thermal internal combustion engines of vehicles with hybrid and micro-hybrid engines equipped with the Stop-and-Start system, makes it possible to considerably reduce wear on the bearings present in said engines, which makes it possible to increase the lifetime of the engine and to increase the time between changes of engine parts. The Applicant company has therefore developed novel lubricant compositions comprising at least one polyalkylene glycol obtained by polymerization or copolymerization of alkylene oxides, including at least one butylene oxide, and also comprising at least one viscosity index improver polymer. Moreover, the quantity of polyalkylene glycol in the lubricant compositions according to the invention is comprised between 1 to 28% by mass, with respect to the total mass of lubricant composition. These particular quantities make it possible to reduce the wear of thermal internal combustion engines. In particular, the compositions according to the invention make it possible to reduce wear on the bearings present in the engines, in particular the engines of vehicles with hybrid engine and vehicles with micro-hybrid engine, including in particular the engines of vehicles with micro-hybrid engine equipped with the “Stop-and-Start” system.
Moreover, the Applicant company has surprisingly found that the combination of these polyalkylene glycols and certain inorganic friction modifiers, in particular organomolybdenum compounds, advantageously makes it possible to reduce wear on the bearings of the engines yet further. Polyalkylene glycols used as lubricant composition additives are known from document WO2011/011656. These compounds have the advantage of being biodegradable and soluble in the four groups of base oils used for the manufacture of lubricant compositions. The document U.S. Pat. No. 6,458,750 describes an engine oil composition with reduced deposit-formation tendency, said composition comprising at least one base oil and at least one alkyl alkoxyalte of formula (I):
wherein
R1, R2, R3 represents independently a hydrogen atom or a hydrocarbon group containing up to 40 carbon atoms,
R4 is a hydrogen atom or a methyl group or an ethyl group,
L is a linker group,
n is an integer ranging from 4 to 40,
A is an alkoxy group with 2 to 25 repeating units, which are derived from ethylene oxide, propylene oxide and/or butylene oxide and comprising homopolymers as well as statistical copolymers of at least two of the said compounds, and
z is 1 or 2.
This composition may also comprise a viscosity index improver polymer. However, this document does not describe a lubricating composition for engines comprising at least one organomolybdenum compounds.
The document EP0438709 discloses an engine oil comprising at least one base oil, at least one polymeric viscosity index improver and at least one product resulting from the reaction of alkyl phenols or bisphenol A with at least one butylene oxide or a butylene/propylene oxide for improving piston cleanliness of automobile engines. However, this document does not describe a lubricating composition for engines comprising at least one organomolybdenum compounds. Moreover, none of these documents describe the use of polyalkylene glycols in a lubricant composition for reducing the wear of thermal internal combustion engines of vehicles with hybrid or micro-hybrid engine and in particular for reducing wear on the bearings.